In recent years, fruit juice manufacturers and the consuming public have become somewhat critical of the standard polyethylene-coated paperboard container currently on the market because of its propensity to absorb and/or lose various essential oils, vitamins and critical flavoring components found in most fruit and citrus beverages. With specific reference to citrus, peel oil readily diffuses through the container's inner polyethylene coating and becomes absorbed by both the polyethylene and the paperboard substrate itself, thereby significantly detracting from the fruit juice's flavor and quality. In addition, oxygen readily penetrates the container and oxidizes various flavoring components found in the juice, including the l-ascorbic acid (vitamin C) which can decompose into other materials, thereby reducing the product's l-ascorbic acid (vitamin C) content and also adversely affecting the juice's flavor. A more detailed description of the problems associated with using polyethylene-lined cartons to store citrus beverages is set forth in U.S. Pat. No. 4,529,606, which is hereby incorporated herein by reference.
One well known attempt to solve the above-identified problem has been to make containers from a laminate having an intermediate metal foil layer, usually aluminum. An example of a paperboard/aluminum foil/polyethylene laminate is described and disclosed in U.S. Pat. No. 3,365,111, which is hereby incorporated herein by reference. As described therein, the intermediate aluminum foil layer does provide a good barrier against oxygen from entering the container. In addition, the foil does inhibit the beverage's essential oils from being absorbed by the paperboard base. Nonetheless it has been found that the beverage-contacting polyethylene layer continues to readily absorb an unacceptable amount of the beverage's essential oils and flavoring components.
Another attempt at solving the above-identified problem of retaining essential oils, vitamins (particularly vitamin C) and flavoring components found in various packaged citrus beverages is disclosed in U.S. Pat. No. 4,513,036, which issued on Apr. 23, 1985. The barrier laminate disclosed therein comprises, from the outer surface to the inner surface, a paperboard substrate, a web of propylene polymer coated thereon, and a web of olefin polymer overlying the propylene polymer web. Preferably, the paperboard substrate is also coated on its external surface with a web of heat sealable olefin polymer. However, the exemplary data set forth in the patent disclose that over a six week storage period containers made from such a laminate only provide a marginal improvement in retaining essential oil, specifically d-limonene (a major component in peel oil), when contrasted with a standard polyethylene/paperboard/polyethylene laminate container. In particular, the preferred carton structure exhibited a 39.5 percent loss of essential oil vs. a 60.5 percent loss of essential oil for the standard polyethylene/paperboard/polyethylene carton. The preferred carton structure exhibited an even smaller degree of improvement over the standard carton with respect to retention of vitamin C. In particular, the standard polyethylene/paperboard/polyethylene carton exhibited an 84 percent loss of vitamin C at the end of the six week period, while the preferred carton structure exhibited a 71 percent loss over the same period.
It is generally known in the art that a polyester, such as PET (polyethylene terephthalate), has excellent flavor barrier properties. It is therefore not surprising that PET is used extensively in forming blow-molded bottles to contain beverages such as soft drinks. There has also been some success with indirectly laminating PET to paperboard for use in other applications. For example, U.S. Pat. No. 4,455,184 discloses a method of coextruding a PET layer with an intermediate layer of polymeric adhesive to a paperboard substrate. The resultant laminate is useful in constructing shallow ovenable trays, it being well known in the art that it is extremely difficult to heat-seal PET to PET as would be the case in trying to make a PET-lined, gable-top carton such as that disclosed in U.S. Pat. No. 3,120,333. The major difficulty in making such a sealed carton lies with the extreme temperatures (around 500.degree. F.) required to heat seal PET to PET. Indeed, at such extreme temperatures, common paperboard severely discolors and even starts to burn. To make matters even worse, PET crystalizes when cooled from such temperatures necessary to create a PET/PET heat seal, thereby making it virtually impossible to obtain commercially-reliable, liquid-tight seals.
Recently, a new polyester resin generally referred to as PET-G (glycol-modified polyethylene terephthalate) has been introduced in the marketplace. For example, U.S. Pat. No. 4,572,340 issued to Ferrante et al. on Feb. 18, 1986 discloses an ovenable self-opening bread dough container. In a preferred embodiment, glycol-modified polyethylene terephthalate is suggested for use as a dough-contacting liner over a substrate which may be comprised of paperboard.
Although the industry acknowledges PET-G's good clarity, barrier properties, and processability, the industry has failed until now to recognize that unlike PET, PET-G can be laminated to substrates such as paperboard without the need of intermediate adhesive layers or pre-treating the substrate with special materials or processes. Of further significance is the fact that unlike layers of PET, it has been found that layers of PET-G can be heat-sealed to one another to form a liquid-tight seal by using conventional heat-sealing techniques and equipment. Because of the inherently more amorphous nature of PET-G, required sealing temperatures are significantly lower than those required to seal PET. Moreover, stronger bonds can be achieved with PET-G due to the fact it has less tendency to crystalize upon cooling. In addition the lower bonding and sealing temperatures of PET-G minimize the chance of browning the paperboard substrate. Accordingly, the present invention is directed to a vast array of laminates that are extremely useful in making economical cartons and containers that not only do an excellent job of preserving and maintaining flavor volatiles found in the beverage contained therein, but also exhibit commercially acceptable, liquid-tight seals.
In light of the above, a principal object of the present invention is to provide various economical barrier laminates that are useful in making containers exhibiting superior retention and preservation of essential oils, vitamins and flavoring components found in various liquids contained therein, said laminates comprising a base substrate such as paperboard, and a beverage-contacting innermost layer of heat-sealable PET-G. In other particularly preferred embodiments, the laminates also include intermediate layers of oxygen-barrier materials such as aluminum foil, PET, nylon, polyvinylidene chloride (PVDC), and ethylene-vinylalcohol copolymer (EVAL).
Another principal object of the present invention is to provide economical containers that are constructed from said superior barrier laminates for the retention of essential oils, vitamins and flavoring components found in many liquids, particularly peel oil and l-ascorbic acid (vitamin C), as normally found in citrus beverages. As will be appreciated, seals which are both gas and liquid tight are essential if such barrier laminate containers are to function in their intended manner.
Another object of the present invention is to provide a barrier laminate blank comprising a substrate having a product-contacting innermost layer of PET-G that can be readily folded and conventionally heat sealed to form a liquid-tight, economical carton that substantially preserves the essential oils, vitamins and flavoring components found in the beverage stored therein for a period of time which approximates the amount of time most beverage products remain in the carton prior to use by the consumer, e.g., typically about six weeks.
It is a further object of the present invention to provide a method of making superior barrier laminates such as PET-G/paperboard/PET-G and PET-G/PET/paperboard/PET-G that are useful in making conventionally heat-sealed containers for storing various beverages that contain essential oils, vitamins and flavoring components.